As evidenced from this image, clouds are a very significant part of the Earth's appearance from space and contribute to its albedo, a measure of what fraction of the sun's radiation is absorbed by the Earth surface. Averaged over the whole Earth and whole year, cloud cover is believed to reduce the radiation received at the Earth's surface by 27.6 W/m2.[1] For comparison, the forcing associated with climate change between 1750 and 2000 is about 1.2 W/m2[1]; hence, small variations in cloud cover are considered to be potentially significant source of climate variability. Unfortunately, clouds are considered to be one of the primary sources of uncertainty in current climate models, largely because the evolution of cloud cover depends on small scale processes and trace gases that are not easily modelled with existing computational techniques [2].

Some researchers have suggested that cloud cover is modulated by changes in cosmic rays reaching the Earth, which is in turn affected by solar variability.[2][3] If true, it would mean the effects of solar variability on Earth's climate have been underestimated. However, the lack of any trend in solar activity during the last 50 years is generally taken as convincing evidence that solar variation, with or without a cloud feedback, has not contributed substantially to recent global warming.[4]

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